Thread rolling is a principal activity of bolt and nut manufacture. Most commonly, thread rolling is achieved by forcing at least two dies into a bar having a smooth surface and, by causing rotation of that bar with respect to the dies, metal is displaced to create a thread form on or in the bar itself. The dies employed in such procedures are typically made from hardened steel and have a suitable thread form machined into them such that, as they are forced into the bar, metal is displaced to create the desired thread form in the bar itself. The dies are typically either circular or flat. Circular dies usually have either two or three circular dies arranged such that there is a space between the dies to allow the bar to pass therethrough. This thread rolling process is known as “through rolling”, since the thread form is progressively formed as the bar passes through or across the dies themselves. If the thread rolling process utilises flat dies, such these are usually used in pairs with each die typically being of the order of 150 mm wide, and being spaced apart to allow the bar to pass through the gap existing therebetween. The flat dies press into the bar over the whole width of the die. This process is known as “plunge rolling”. Plunge rolling is a faster process than through rolling. However, both plunge rolling and through rolling are collectively known as “Cold Rolling of Threads”.
Cold rolling of threads necessitates the displacement and flow of metal. Some metal is displaced away from the points of the dies, and some metal is displaced upwards to form the new high points of the thread on the bar. This cold rolling and displacement of metal causes the metal in the thread to become harder. Hence, cold rolled threads are normally stronger than machined threads. This process of the steel or the like material becoming harder is known as “Cold Working”.
However, cold working of steel can also cause the metal, and in particular some grades of steel, to become brittle. Cracking can occur at the root or base of the formed thread. This can lead to a weakness in the formed thread and be a source of premature thread failure. For example, where the threaded section of a rock bolt protrudes from the roof of a mine or tunnel, it can be subject to damage through being hit by heavy machinery passing along the roadway. If the threaded section is brittle, premature failure of the rock bolt can occur.
Cold rolling of threads can normally only be performed on bars, having a smooth surface, or the like members. Therefore, bars with deformations on them require that the deformations be removed before a thread rolling process can be undertaken. By way of example only, rock bolts produced from hot rolled bars with deformations on them could typically have a core dimension of 21.7 mm, having a maximum diameter across the deformations of 24 mm. These deformations could be removed either by bar peeling or by swaging prior to thread rolling, such that a bar, having a smooth surface, with a diameter of, for example, 21.6 mm, would be produced. A thread could then be cold rolled onto such a bar and, in this case, it would typically be an M24 thread (i.e. a metric 24 mm thread).
An M24 thread has a pitch of 3 mm. That is, one revolution around the thread causes axial movement along the axis of the bar of 3 mm. The pitch of the thread determines its mechanical advantage and the angle that the threads form with the longitudinal axis of the bar. A 3 mm thread pitch provides excellent mechanical advantage for rock bolts and a tensile load of between 2 and 10 tonnes can be generated in such rock bolts, depending on the torque applied by the drilling machine being employed.
A very fine thread provides even greater mechanical advantage, but is more susceptible to thread damage. This is especially the case for rock bolts and concrete tie rods, which are used in rugged environments. Conversely, coarse threads are less susceptible to damage but provide poor mechanical advantage.
Threads may can also be formed on bars using what is known as a hot rolling process. As a bar is being formed in a hot rolling mill, synchronised rolls can be used to press a thread form into opposite sides of a bar. The ribs which are so formed protrude from the bar and typically form a discontinuous thread around and along the bar. Some advantages associated with a hot rolled thread include:                the thread is not affected by cold working;        the tensile strength and elongation characteristics of the bar are uniform all the way along the bar, unlike cold rolled threaded bars where the root diameter of the threaded section is the weakest part of the bar;        the bar and the thread are less susceptible to damage because the thread itself is coarse;        the thread ribs are an integral part of the bar and are less likely to be affected by cracking occurring at the base of the ribs;        threads can be formed in materials, particularly high tensile strength steels, that would be unsuitable for thread cold rolling;        the process of the hot rolling of threads is very fast and economical and does not require a secondary processing operation, unlike cold thread roiling procedures which require bar peeling or swaging in addition to cold thread rolling.        
However, a disadvantage is that hot rolled threads are usually very coarse. For example, hot rolled threads would typically have a 10 mm or greater pitch dependent upon bar diameter. The main reason for having a coarse hot rolled thread is that, although a fine thread form could be machined into the rolls used in a hot rolling mill, such a fine thread form would wear out very quickly. The fine machining and sharp points required in a roll to form a fine thread would wear or break as the hot bar passed through the rolls at the speeds normally employed, which may be up to, for example, 10 meters per second.
For this reason, the thread ribs also tend to be wide and have a “flat” crest to the thread form typically 1 mm wide or greater. This coarse thread on hot rolled threads has the advantage of making the thread very robust and less susceptible to damage, but on the other hand provides poor mechanical advantage and makes it difficult to apply high tensile loads in bars and bolts thus formed. Typically, hot rolled threaded bars which have diameters of 26.5 mm, 32 mm and 36 mm, respectively, have pitches of 13 mm, 16 mm and 18 mm, respectively.
A hot rolling process involves passing a billet of hot steel through a series of rolling stands to progressively reduce the size of the billet down to the desired diameter for of the final product. Typically, billets may be from 90 mm×90 mm up to 150 mm×150 mm and up to 12 m long, which are heated up to approximately from 900 to 1100° C. and are then passed through a series of rolls (normally between 10 and 20 pairs of rolls) to progressively reduce the diameter of the billet. As the billet reduces in diameter it increases in length, and hence its speed through the mill also increases. Typically, a billet would enter the first rolling stand at a slow speed of, for example say, 0.5 meters per second and, by the time it has passed through the last rolling stand, it could be travelling at, for example say, 10 meters per second. Such a hot rolling procedure is a very fast and efficient method of manufacture for a wide range of bars and sections.
In the case of rock bolts, the hot rolled thread is formed on the bar in the last rolling stand. Ribs are machined into the rolls as “grooves” in the rolls such that, as the bar is squeezed by the rolls, a male rib would be formed on the bar. Multiple grooves are machined into the top and bottom rolls and each roll is synchronised with the other of each mating pair, such that a thread form is produced on the hot rolled bar. For hot rolled bars produced using presently known technologies, these grooves are spaced and angled to the axis of the bar, such that they form a coarse pitched threaded bar.
It is an object of the present invention to provide a process for the formation of a thread form that has all the advantages of a hot rolled thread, but has a similar pitch to a cold rolled thread, such that it exhibits the mechanical advantages associated with both a hot rolled thread and a cold rolled thread. The process of the present invention seeks to provide a hot rolled threaded member having a relatively fine-pitched thread.
In a typical rock bolt application, the present invention can produce a bar that is simply cut to length and then only a suitable nut and domed ball needs to be attached to the bar to produce a finished rock bolt. No additional post-rolling manufacturing is required.
An additional significant advantage of the process of the present invention is that it allows multiple hot rolled threaded bars to be joined together, using one or many couplers, depending upon the number of bars to be joined together.
For a conventional coupled bar or coupled rock bolt arrangement, the ends of two threaded bars may be screwed into each end of a female threaded coupler. The coupler is of sufficient length to engage enough threads on the bar, and is designed to be stronger in tension than the tensile strength of the bar such that when two bars are each screwed firmly into the coupler, the coupled joint of the two bars is stronger than the solid bar itself. By using multiple couplers, it is possible to form a very long solid bar and this has significant applications in underground mining and tunneling applications.
This form of coupled bar or coupled rock bolt is well known prior art and is used where long bolts are required for geotechnical or other reasons. However, where long bolts are required, cables bolts are normally used rather than coupled solid bars. This is primarily for two reasons.
Firstly, cables are made from much higher tensile strength steel than solid bolts (typically 1500 MPa for cables compared to 800 MPa for solid bolts for their respective ultimate tensile strengths) and this enables cables to be produced with both high tensile strength (typically 50 to 75 tonnes for mining applications) and reasonable weight (typically less than 5 kgs per meter).
Secondly, it is possible to make very long cables, which can be bent to fit into confined spaces in underground tunnels and mines and still be installed to provide long bolt support.
Conventionally, coupled solid bolts can compete with long cables bolts but, to obtain the same high tensile strength as cables, it is necessary to use a larger diameter solid bar. This means that a different and larger diameter solid bar must be produced to be used as a coupled bolt to compete with cables. This requires an additional product to be made by the steel mill to make a large diameter bar for coupled bolt applications only and this will not be as common as smaller diameter solid bars used for general rock bolting applications.
The weight of a larger diameter solid bar for a coupled bolt is not usually a problem, since drilling machines can easily push multiple solid coupled bars up a hole. The fact that solid coupled bars can be pushed is a major advantage and drilling machines can easily push them up holes and through multiple resin cartridges, which is more difficult to do with a flexible cable or cable bolt.
The other major advantage of solid coupled bars is that that they can be produced with a hot rolled ribbed external profile and this can provide a high bond strength with resin or grout. This is known as a rock bolt's load transfer capacity and the higher the load transfer capacity, the more effectively the rock bolt will support the tunnel or mine roadway. Cables cannot provide such a high load transfer capacity as hot rolled ribbed bars or bolts.
For conventional solid coupled bolts, the top of the coupled bolts at the top of the hole is anchored either by resin or by a mechanical anchor and the rest of the coupled bolts can be grouted either with cement, resin or a polyurethane resin (PUR). The grout is normally pumped up from the bottom collar of the hole and flows up around the bolts and around the couplers. Alternatively, a grout tube can be used where the grout is pumped up the tube to the top of the hole and fills up the cavity between the bolt and the hole with grout.
Whether a grout tube is used or not used with a conventional coupled bolt, there must be sufficient clearance left between the outside diameter of the coupler and the diameter of the hole to allow grout to flow easily around the coupler and fill up the cavity between the bolt and the hole. This necessitates the use of a larger diameter hole than is necessary just to fit the coupler up the hole. The problem is further exacerbated if, for example, a 20 mm diameter grout tube has to fit around the outside of the coupler.
Conventional coupled bolts, therefore, have the following disadvantages. They require the use of larger diameter bar than standard rock bolts in order to generate similar tensile capacity as cables. They also require the use of couplers, where there must be sufficient clearance between the outside of the coupler and the borehole wall to allow grout and or a grout tube to pass around the coupler.
The new thread form of the present invention further allows a new coupled rock bolt or coupled bar to be used in a manner as described hereinafter in more detail.
It should be noted that the couplers and assembled bars described can be used when any threaded bar according to the present invention is joined to another bar, for example in concrete reinforcing bars, foundation tie down bolts, formwork tie bars and small diameter flexible bars making up a larger assembled bolt. However, the present invention is not so limited.
The invention herein is described with particular reference to the manufacture of rock bolts, but it should be understood that the invention is not to be considered to be limited in any way to any particular or preferred embodiment or embodiments described. Rather, the present invention could be equally applied to any threaded elongate member. The invention is particularly, but not exclusively, applicable to hot rolled threaded bars but is not so limited.